Setting or updating a profile image
Some email clients will show a profile image next to your emails. This profile image cannot be set in the email itself, and it's not currently possible to set your logo in one place so that it displays in all email inboxes. In most cases, it works by setting up an account with each email client using your business email address.
Gravatar
Gravatar is a free service that associates an image with your email address. It is supported by a handful of 3rd party email clients like Superhuman, Thunderbird and Spark Mail so setting up an account with Gravatar would usually allow for your image to be displayed by those email clients. You will need to sign up to Gravatar (using the email address you send your emails with) or access your existing account to manage the image.
If your subscribers have Gmail accounts but view their email using the Spark Mail app, they'll see your Gravatar.
If you have a Google profile associated with your business email address, you can set or update your profile picture on your about me screen. If not, you can create an account using the email address you're sending from.
If you've updated both of these and you're not seeing a profile image in the email clients mentioned, make sure you have set up SPF and DKIM on your domain. Verifying your domain is the easiest way to do this.
Other email clients
If you'd like your profile image to display in other email clients, like Outlook or Yahoo, you'd need to set up an account using your business email address with each of them.
Another way would be to use BIMI - Brand Indicators for Message Identification. It's an extra way of authenticating your email, which also allows you to display your logo in many different email clients. However, please keep in mind that BIMI is not yet universally supported.